The present invention is related to co-pending U.S. application entitled "PREHEATING APPARATUS FOR DIESEL ENGINES" filed on Oct. 9, 1980 in the name of Yoji KATO and assigned to the same assignee.
The present invention relates to a preheating apparatus for Diesel engines, which is designed so that the glow plugs of an engine are provided with a rapid temperature rise characteristic and the air drawn into the engine is also heated with a view to improving the starting characteristic of the engine.
In the past, Diesel engines of the pre-combustion chamber type have been mostly equipped with glow plugs for engine starting facilitating purposes so that during the engine starting the glow plugs are heated red hot and the fuel is contacted with the glow plugs to burn it and thereby facilitate the starting of the engine. There has been a desire that the time required for heating the glow plug red hot (hereinafter referred to as a "glow plug preheating time") and the time required for starting the engine through the operation of the starter (hereinafter referred to as a "cranking time") be reduced as far as possible.
In view of these background circumstances, systems have been proposed in which as shown, for example, in Japanese Utility Model Publication No. 48262/77, in order to reduce the preheating time, a resistance unit having a very high positive temperature coefficient of resistance, such as one made from molybdenum disilicide, is connected in series with a parallel circuit of glow plugs so that during the starting period a large current is supplied to the glow plugs through the action of the resistance unit and the temperature of the glow plugs is increased rapidly.
However, the prior art apparatus of the above type has the following disadvantages.
(1) While the molybdenum disilicide resistor constituting the resistance unit increases in resistance value linearly with temperature, the ratio of change in resistance between the lower and higher temperature ranges is about 6 times so that the resistance value at the normal temperature becomes 1/6 of the resistance value in the higher temperature range and thus the normal temperature resistance value cannot be set so small. This limits the amount of starting current that can be supplied to the glow plugs and the resulting preheating time is 15 seconds as compared with the conventional time of 20 seconds, showing only an improvement by 25%. PA1 (2) When the current is being supplied to the glow plugs, the current is always flowing to the molybdenum disilicide resistance unit so that the resistance unit always generates heat and the resistance unit is always kept at elevated temperatures by this heat generation, causing its characteristics to change greatly with time. Thus, there is a disadvantage that even if the initial resistance value is preset accurately, the resistance value is subject to variation with the accumulated period of service time of the glow plugs, thus giving rise to such troubles as burnout failure, faulty heat generation, etc., of the glow plugs.